In recent years, since substitution of petroleum resources and environmental consciousness have been increased, there has been a focus on materials using recyclable natural fibers. Among the natural fibers, a cellulose fiber having a fiber diameter of 10 to 50 μm, particularly, cellulose fiber (pulp) derived from a tree has been widely used mainly as a paper product so far.
As for the cellulose fiber, microfibrous cellulose has also been known having a fiber diameter of equal to or less than 1 μm. A sheet containing the microfibrous cellulose has advantages such as high mechanical strength and high air permeability and the application is considered for various purposes.
As a method for producing a microfibrous cellulose-containing sheet, in PTLs 1 to 3, papermaking from microfibrous cellulose using a wire for papermaking in the related art is disclosed. However, since the microfibrous cellulose cannot be sufficiently trapped by using the wire for papermaking in this method, sufficient filtering efficiency cannot be obtained and the yield rate is low. In particular, microfibrous cellulose having an average fiber diameter of 2 to 200 nm and an average fiber length of 0.01 to 100 μm can be hardly trapped by the wire for papermaking in the related art and cannot be formed into a sheet.
In PTLs 4 to 6, a method in which a microfibrous cellulose suspension is flow-casted into glass, a resin plate, a metal plate and the like and dried to be formed into a sheet is disclosed. However, this method requires energy and time for drying when forming the sheet and included increased costs.
In PTLs 7 and 8, there is suggestion that a filter fabric having compactness (a predetermined range of air permeability), a non-woven fabric including organic polymer fiber, a textile, or a porous membrane filter fabric including an organic polymer is used as a wire for papermaking in order to industrially produce a microfibrous cellulose-containing sheet. However, the filter fabric, textile and non-woven fabric cannot sufficiently trap microfibrous cellulose and the yield rate is low. In addition, since recycling is difficult and is expensive after the filter fabric, textile and non-woven fabric are used in filtering once, the filter fabric, textile and non-woven fabric are disposable, and manufacturing costs are increased. When the filter fabric, textile and non-woven fabric are used as the wire for papermaking, the surface quality of the obtained microfibrous cellulose-containing sheet is insufficient.
In addition, while a method for using filter paper having a large number of pores, or a membrane filter which is a film-shaped filter and the like is known, as a method in which the microfibrous cellulose suspension is filtered, dehydrated and formed into a sheet, there is a tendency to easily cause clogging due to the microfibrous cellulose and an increase of filtration time.